Printed, self-aligned, top gate thin film transistor

ABSTRACT

A self-aligned top-gate thin film transistor (TFT) and a method of forming such a thin film transistor, by forming a semiconductor thin film layer; printing a doped glass pattern thereon, a gap in the doped glass pattern defining a channel region of the TFT; forming a gate electrode on or over the channel region, the gate electrode comprising a gate dielectric film and a gate conductor thereon; and diffusing a dopant from the doped glass pattern into the semiconductor thin film layer.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/813,161, filed Jun. 12, 2006, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a printed, self-aligned, top-gate thinfilm transistor (TFT), in which metal-containing inks can be used forprinting the gate metal. In preferred embodiments, the metal-containingink comprises metal nanoparticles. The present invention advantageouslyrequires little or no high-temperature or laser activation processingafter printing the metal ink.

DISCUSSION OF THE BACKGROUND

In a conventional top-gate TFT process, alignment between the gate andthe source/drain regions is ensured by first patterning the gatematerial and using it as a mask for dopant implantation and/oractivation. This approach poses challenges regarding the choice of thegate metal because it needs to be either reflective to (UV) laserirradiation (e.g., Al) or compatible with thermal dopant activation attemperatures higher than 600° C. (e.g., doped polysilicon or arefractory metal such as Mo, Pd or W).

Conventional printing technologies (e.g., ink-jetting) can beadvantageous for manufacturing electronic devices, due to the highthroughput of printing processes relative to photolithography. However,high-resolution printing techniques are typically limited with respectto the width of a printed line (about 10 μm or larger), due to therelatively large volume of the drops.

Thus, it would be desirable to develop a process for making TFTs thatcan form small (e.g., <10 μm) line-width structures such as gates usingprinting technology, and/or that is not restricted to certain gatematerials such as aluminum, refractory metals or doped polysilicon.

SUMMARY OF THE INVENTION

One object of the invention is to provide a method of forming a thinfilm transistor (TFT), comprising: forming a semiconductor thin filmlayer; printing a doped glass pattern thereon, a gap in the doped glasspattern defining a channel region of the TFT; forming a gate electrodeon or over the channel region, the gate electrode comprising a gatedielectric film and a gate conductor thereon; and diffusing a dopantfrom the doped glass pattern into the semiconductor thin film layer.

Another object of the invention is to provide a thin film transistor(TFT), comprising: semiconductor thin film layer; at least part of adoped glass pattern thereon, wherein at least two portions of the dopedglass pattern define a gap over a channel region of the TFT; a gateelectrode on or over a channel region of the semiconductor thin filmlayer, the gate electrode comprising a gate dielectric film and a gateconductor thereon; and dopant-containing regions in the semiconductorthin film layer on opposed sides of the channel region.

Another object of the invention is to provide a method of forming a thinfilm structure, comprising: forming a semiconductor thin film layer;printing a doped glass pattern thereon, a gap in the doped glass patterndefining a channel region of the TFT; and diffusing a dopant from thedoped glass pattern into the semiconductor thin film layer.

Another object of the invention is to provide a thin film structure,comprising: a semiconductor thin film layer; at least part of a dopedglass pattern thereon, wherein at least two portions of the doped glasspattern define a gap over a channel region; and dopant-containingregions in the semiconductor thin film layer on opposed sides of thechannel region.

In one respect, the present invention recognizes that the space betweentwo lines may be smaller than the minimum width of an inkjet-printedline because the space between lines is mainly determined by inkplacement accuracy plus the accuracy and addressability of themechanical stages of the printer. Therefore, first printing asource/drain pattern which defines the location of subsequentlydeposited gate metal enables one to fabricate high performance printedtop gate TFTs with channel widths <10 μm.

The present invention is therefore directed towards the manufacturing ofthin film transistors and circuits thereof on various substratesincluding, but not limited to, glass (e.g., quartz) sheets or slips,plastic and/or metal foils, sheets or slabs, silicon wafers, etc., allof which may carry one or more buffer layers (such as silicon and/oraluminum oxide). Applications include but are not limited to displays,RF devices, sensors, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-E show cross-sectional views at various stages of an exemplaryprocess flow process for making a printed self-aligned top gate device,showing a first dopant drive-in technique.

FIGS. 2A-E show cross-sectional views of another exemplary process flow,including gate oxide formation after the printing of doped glass.

FIGS. 3A-C show cross-sectional views of a process flow that reducespotential out-diffusion from the doped glass during gate dielectricformation and (optional) further processing.

FIGS. 4A-D show cross-sectional views of a process flow for etching thedoped glass pattern and forming source/drain contacts and interconnects,using the metal gate as a mask.

FIGS. 5A-E show cross-sectional views of an alternative process flow forforming source/drain contacts and interconnects.

FIGS. 6A-C show cross-sectional views of an alternative process flow inwhich a dielectric material over the metal gate is used as a mask whenetching the doped glass pattern and forming source/drain contacts andinterconnects.

FIGS. 7A-D show cross-sectional views of an alternative process flowwhich uses a printed interlayer dielectric as a mask for etching thedoped glass and exposing the doped polysilicon source/drain regions forsubsequent contact/interconnect formation.

FIGS. 8A-D show cross-sectional views of a further alternative processflow which uses a printed interlayer dielectric as a mask fornon-selectively etching the doped glass and exposing the dopedpolysilicon source/drain regions for subsequent contact/interconnectformation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The development of printed refractory metal or Al gates in self-alignedtop-gate TFTs poses a significant challenge, which the present inventionavoids by first patterning a layer defining the source/drain regions,then activating the dopant (e.g., by annealing at high temperature or bylaser activation), and subsequently depositing the gate metal precursorink. In a preferred embodiment, simple noble metal inks containingsilver or gold can be used for printing the gate metal, since nohigh-temperature or laser activation process step is requiredafterwards. The present TFTs may be capable of operating at GHzfrequencies and may benefit from (1) narrow channel widths, (2) sourceand drain terminals that are self-aligned to the gate with a smallamount of overlap between, and/or (3) high carrier mobility.

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing disclosure numerous specific details are given to provide athorough understanding of the invention. However, it will be apparent toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-known methods,procedures, components, and circuits have not been described in detail,to avoid unnecessarily obscuring aspects of the present invention.

For the sake of convenience and simplicity, the terms “coupled to,”“connected to,” and “in communication with” (and variations thereof)refer to direct or indirect coupling, connection or communication,unless the context clearly indicates otherwise. These terms aregenerally used interchangeably herein, and wherever one such term isused, it also encompasses the other terms, unless the context clearlyindicates otherwise. In the present disclosure, the term “deposit” (andgrammatical variations thereof) is intended to encompass all forms ofdeposition, including blanket deposition, coating, and printing, unlessthe context clearly indicates otherwise. Furthermore, with regard tocertain materials, the phrase “consisting essentially of” does notexclude intentionally added dopants, which may give the material towhich the dopant is added (or the element or structure formed from suchmaterial) certain desired (and potentially quite different) physicaland/or electrical properties. The term “(poly)silane” refers tocompounds or mixtures of compounds that consist essentially of (1)silicon and/or germanium and (2) hydrogen, and that predominantlycontain species having at least 15 silicon and/or germanium atoms. Suchspecies may contain one or more cyclic rings. The term “(cyclo)silane”refers to compounds or mixtures of compounds that consist essentially of(1) silicon and/or germanium and (2) hydrogen, and that may contain oneor more cyclic rings and less than 15 silicon and/or germanium atoms.The term “hetero(cyclo)silane” refers to compounds or mixtures ofcompounds that consist essentially of (1) silicon and/or germanium, (2)hydrogen, and (3) one or more dopant atoms such as B, P, As or Sb thatmay be substituted by a conventional hydrocarbon, silane or germanesubstituent and that may contain one or more cyclic rings. Also, a“major surface” of a structure or feature is a surface defined at leastin part by the largest axis of the structure or feature (e.g., if thestructure is round and has a radius greater than its thickness, theradial surface[s] is/are the major surface of the structure; however,where the structure is square, rectangular or oval, the major surface ofthe structure is typically a surface defined by the two largest axes,generally the length and width).

Representative cyclosilane compounds of the formula (AH_(z))_(k) whereinA is Si, z is 1 or 2 (preferably 2) and k is from 3 to 12 (preferably 4to 8) and an exemplary method for their preparation are described ingreater detail in copending application Ser. No. 10/789,317, filed Feb.27, 2004, the relevant portions of which are incorporated herein byreference. Representative hetero(cyclo)silane compounds, doped silaneintermediates, exemplary methods for their preparation, and techniquesfor determining and/or controlling the dopant levels in the precursorinks and active films are described in greater detail in copendingapplication Ser. Nos. 10/950,373, 10/949,013 and 10/956,714,respectively filed on Sep. 24, 2004, Sep. 24, 2004 and Oct. 1, 2004, therelevant portions of which are incorporated herein by reference. Oligo-and polysilane compounds are disclosed in U.S. Provisional Appl. Nos.60/850,094 and 60/905,403, filed Oct. 6, 2006 and Mar. 5, 2007,respectively, the relevant portions of which are incorporated herein byreference.

Typically, although not necessarily always, liquid-phase semiconductorinks further, comprise a solvent, preferably a cycloalkane (e.g., amono- or bicycloalkane). Thus, when using an ink comprising orconsisting essentially of a Group IVA element source (such as asilane-based precursor to Si or doped Si), forming the semiconductorlayer may further comprise drying the liquid-phase precursor ink afterdeposition. See copending U.S. application Ser. Nos. 10/616,147,10/789,317 and 10/789,274, respectively filed Jul. 8, 2003, Feb. 27,2004 and Feb. 27, 2004, the relevant portions of each of which areincorporated herein by reference.

After deposition (and generally, at least some drying), a semiconductorlayer is generally cured by heating, as described above in copendingU.S. patent application Ser. No. 10/789,274 and 10/949,013, respectivelyfiled on Feb. 27, 2004 and Sep. 24, 2004, the relevant portions of whichare incorporated herein by reference) to form an amorphous, hydrogenated(doped) silicon (a-Si:H) layer. When a semiconductor layer originates oris formed from a (poly)silane, (cyclo)silane and/or hetero(cyclo)silane,the curing/heating step may remove unwanted precursor/ink components orbyproducts such as volatile carbon-containing species, or reduce thehydrogen content of the a-Si:H layer (which is particularly advantageousif laser crystallization is to be used after semiconductor filmformation). When a semiconductor layer originates or is formed from ahetero(cyclo)silane, the curing/heating step may also activate part ofthe dopant in the hetero(cyclo)silane, but in many embodiments, dopantactivation may be more likely to occur during laser crystallization.

Also, a doped semiconductor layer may be deposited by local printing ofthe liquid semiconductor precursor ink directly onto a structure such asthe gate metal (e.g., to form a silicide), dielectric layer (e.g., agate dielectric) and/or semiconductor layer (see, e.g., copending U.S.patent application Ser. Nos. 10/949,013, 11/084,448 and 11/203,563,filed on Sep. 24, 2004, Mar. 18, 2005 and Aug. 11, 2005, respectively,the relevant portions of which are incorporated herein by reference).This approach to forming MOS TFT structures may be cost effective due to(i) the efficient usage of the semiconductor precursor materials and(ii) the combination of semiconductor deposition and patterning into oneprinting step.

Blanket deposition may comprise, e.g., evaporation, physical vapordeposition, sputtering, or chemical vapor deposition, as is known in theart. Alternatively, blanket deposition may comprise spin-coating an inkcomprising, e.g., a (cyclo)silane, (poly)silane or metal nanoparticles(which may be passivated) and a solvent, and curing the ink (see, e.g.,U.S. Pat. No. 6,878,184 and U.S. patent application Ser. No. 10/749,876,filed Dec. 31, 2003, the relevant portions of which are incorporatedherein by reference). Metals that may be deposited by such methodsinclude elemental metals, such as aluminum, titanium, vanadium,chromium, molybdenum, tungsten, iron, nickel, palladium, platinum,copper, zinc, silver, gold, etc.; conventional alloys of such elements,such as aluminum-copper alloys, aluminum-silicon alloys,aluminum-copper-silicon alloys, titanium-tungsten alloys, Mo—W alloys,aluminum-titanium alloys, etc.; and electrically conductive metalcompounds, such as the nitrides and silicides of elemental metals (e.g.,titanium nitride, titanium silicide, tantalum nitride, cobalt silicide,molybdenum silicide, tungsten silicide, platinum silicide, etc.). Inother embodiments, the blanket depositing step may comprise spin-coatingan ink containing the metal-containing material, the metal-containingmaterial may comprise metal nanoparticles and/or organometallicprecursors of one or more of the metals disclosed above. Also, whetherthe metal ink is blanket-deposited or printed, the method may furthercomprise the step of curing or annealing the metal, organometallicprecursor(s) and/or metal nanoparticles before or after the step oflaser patterning.

This application describes the design of, and several process flows forfabricating, a printed, self-aligned, top gate TFT. The process flowleverages a printed doped glass in at least one of three ways:

-   -   A printed doped glass provides a source of dopants for        source/drain doping;    -   The printed doped glass defines a space for gate metal and        ensures close (e.g., reasonably acceptable) alignment of the        gate to the source/drain regions/terminals; and/or    -   The printed doped glass functions as an interlayer dielectric; a        further interlayer dielectric may be formed over the doped glass        pattern and the gate electrode. (In one embodiment, removing        portions of the doped glass pattern leaves remaining portions of        the doped glass pattern under an interlayer dielectric film.)

The present invention enables formation of gate line widths of less than10 um, generally by forming a transistor gate in the space betweenprinted structures. However, as drop volumes are expected to decreaseand printing resolution is expected to increase in the future inwidely-used printing techniques such as ink-jetting, gravurelithography, and offset lithography, the space between such printedstructures is also expected to decrease, and the invention will continueto enable formation of gate widths less than the corresponding minimumwidth of a conventionally printed structure.

The invention, in its various aspects, will be explained in greaterdetail below with regard to exemplary embodiments.

Formation of a Self-Aligned Source/Drain Gate Structure

Dopant Drive-In through Gate Dielectric

Referring to FIGS. 1A-E, an exemplary process flow is shown. Theexemplary process of FIGS. 1A-E forms the gate dielectric first, beforeprinting a doped glass pattern. The transistor channel (e.g., comprisingor consisting essentially of amorphous or polycrystalline silicon) istherefore protected from contamination by dopants from the subsequentlydeposited doped glass.

Referring to FIG. 1A, a physically isolated silicon film is generallyformed by printing or coating a molecular and/or nanoparticle-basedsilicon ink on a substrate 1 and converting it to a (poly)silicon thinfilm 2 (e.g., by heating and/or curing it, and optionally, bycrystallizing it by UV laser exposure, or thermal furnace or RTA anneal[optionally in the presence of a crystallization promoter such as Au,Ni, Al, etc.]). Alternatively, one may conventionally deposit a siliconfilm (e.g., by PECVD, LPCVD, sputtering, etc.), crystallize it asdescribed above, and then pattern the polycrystalline film bylow-resolution photolithography and/or selective etching (or vice versa;i.e., pattern, then crystallize the silicon film). When the silicon filmis crystallized by laser annealing, one may simply remove thenon-irradiated, amorphous portions of the deposited silicon film byselective etching in accordance with known techniques. The substratepreferably comprises a silicon wafer, a glass slip or sheet, or aplastic or metal sheet (which may optionally be stiff or flexible andwhich may have a dielectric layer thereon [e.g., in the case of metal,having a thin oxide layer thereon]).

Substrate 1 generally comprises a conventional mechanical supportstructure. Suitable electrically inert or inactive substrates maycomprise a plate, disc, and/or sheet of a glass, ceramic, dielectricand/or plastic. Alternatively, suitable electrically conductivesubstrates may comprise a wafer, disc, sheet and/or foil of asemiconductor (e.g. silicon) and/or a metal. In the case where thesubstrate comprises a metal sheet and/or foil, the device may furthercomprise an inductor and/or capacitor, and the method may furthercomprise forming an inductor and/or capacitor from the metal substrate.However, any such electrically conductive substrate should have aninsulator layer between it and any electrically active layer orstructure thereover (e.g., semiconductor layer 2), except in a locationwhere electrical contact is to be made from the devices on the insulatorto a structure formed in the metal substrate (e.g., one or more metalpads of an interposer, inductor and/or capacitor for an EAS or RFID tag;see, e.g., U.S. patent application Ser. Nos. 10/885,283, 11/104,375and/or 11/243,460, respectively filed on Jul. 6, 2004, Apr. 11, 2005,and Oct. 3, 2005, and/or U.S. Provisional Application Nos. 60/592,596and 60/617,617, respectively filed on Jul. 31, 2004, and Oct. 8, 2004).Preferably, the substrate comprises a member selected from the groupconsisting of a silicon wafer, a glass plate, a ceramic plate or disc, aplastic sheet or disc, metal foil, a metal sheet or disc, and laminatedor layered combinations thereof. For example, plastic substrates mayfurther contain a planarization layer thereon to reduce the surfaceroughness of the substrate, and glass and plastic substrates may furthercontain a surface energy modifying layer of material thereon to improvethe adhesion and/or control the spreading of a material (e.g., an ink)printed or otherwise deposited thereon (see U.S. Provisional ApplicationNo. 60/919,290, filed on Mar. 20, 2007). In addition, electricallyconductive substrates (e.g., metal foils) generally have an insulatorlayer (e.g., a layer of the corresponding oxide) thereon, except inareas where circuitry formed thereon makes electrical contact with astructure (such as an inductor coil, antenna or capacitor plate) formedfrom the conductive substrate.

Forming the semiconductor thin film layer 2 may include printing asemiconductor precursor ink on substrate 1 to form a pattern, drying theink, curing the ink (e.g., by heating and or annealing the dried ink fora length of time sufficient to cross-link, oligomerize and/or polymerizea silane therein, and/or increase an average molecular weight, increasea viscosity and/or reduce a volatility of the composition), thenpartially or substantially completely crystallizing the semiconductorfilm pattern to form a polycrystalline film. Alternatively, whereapplicable, one may simply print an amorphous film, withoutcrystallizing it. The semiconductor thin film layer 2 generallycomprises one or more Group IV elements, preferably polysilicon orsilicon-germanium. Typical semiconductor layer 2 thicknesses may be fromabout 10, 30, 75 or 100 nm to about 200, 500 or 1000 nm, or any range ofvalues therein. The film thickness may be chosen to optimize theelectrical properties of the transistor.

In various embodiments, semiconductor layer 2 comprises or consistsessentially of a lightly doped inorganic semiconductor material, such asone or more Group IVA elements (e.g., silicon and/or germanium), aso-called “III-V” material (e.g., GaAs), a II-VI (or chalcogenide)semiconductor, etc., further containing a dopant (such as B, P, As orSb) in a concentration of from ˜10¹⁶ to ˜5×10¹⁸ atoms/cm³. Exemplarylightly doped semiconductor films are disclosed in copending U.S.application Ser. No. 10/949,013, filed Sep. 24, 2004 , the relevantportions of which are incorporated herein by reference. In oneembodiment, the semiconductor (transistor channel) layer 2 may belightly doped (e.g., having a dopant concentration of from about 10¹⁶ toabout 5×10¹⁸ atoms/cm³). When formed from a silane-based and/ornanoparticle ink, the lightly doped semiconductor layer 2 may have aconcentration profile (e.g., dopant concentration as a function ofsemiconductor layer thickness) in an amorphous state that issubstantially uniform throughout substantially the entire thickness ofthe semiconductor layer.

Referring to FIG. 1B, a gate dielectric 3 may be formed on thesemiconductor (e.g., (poly)silicon) film 2 by thermal oxidation of thesemiconductor layer 2, or alternatively, by printing or coating asuitable dielectric precursor and converting it to a dielectric film(e.g., liquid phase deposition of a SiO₂ precursor, such as silicic acid[H₂SiF₆] and boric acid) or deposition of other metal oxide(s) (e.g.,titanium, zirconium, and [possibly] hafnium analogs of the silicicacid/boric acid mixture to form TiO₂, ZrO₂, HfO₂, etc.) or conventionalCVD, PECVD, LPCVD or sputter deposition of silicon oxide and/or nitridelayers. As shown in FIG. 1C, a doped glass film 4 is then printed (e.g.,by ink-jetting, gravure printing, etc.) onto the gate dielectric 3. Inone embodiment, the gate dielectric film 3 is formed on an entiresurface of the semiconductor thin film layer 2, then a doped glasspattern 4 is printed thereon. The layout of the doped glass pattern isessentially the same as the intended layout for the source-drainstructures of the TFT.

The gap 5 between the printed doped glass film regions defines theposition of the gate metal. The width and/or spacing of the gap mayrange from 0.1-100 μm (preferably 1-10 μm, and in some embodiments, 1-5μm). Upon high-temperature annealing, the dopant from the doped glassmigrates through the gate dielectric into the polysilicon film, definingthe source/drain regions 6. The temperature for diffusing the dopantfrom the doped glass through the gate dielectric is preferably lowerthan 1100° C., but at least about 700° C., for a length of timesufficient to dope the source/drain terminals, but leave channel region7 undoped.

Preferably, the process of printing the doped glass utilizes a tool thatcan simultaneously deposit N- and P-type doped glass in different areasof the circuit. One embodiment utilizes, for instance, an inkjet printerwith at least two inkjet heads on the same gantry, which are separatedby a distance matching or corresponding to the distance (e.g. a multipleof such distance) between the areas of N- and P-type transistors in thecircuit. The two inkjet (IJ) heads may correspondingly be connected toreservoirs of N- and P-type doped glass precursor (e.g., an inkcomprising a doped spin-on glass precursor or other formulation) and maydispense N- and P-type doped glass in different areas of the circuit inthe same printing pass.

Examples of precursors for the doped glass include conventionalspin-on-dopant (SOD) formulations and customized versions with increasedviscosity (e.g., “customized” by replacing or diluting the solvent inthe conventional formulation with a similar or compatible solvent ofhigher viscosity), doped molecular silicon ink formulations which can beoxidized at low temperatures (e.g. ≦400° C.) after deposition (e.g.cyclic, linear or branched silane oligomers or polymers with dopantsubstituents, such as cyclo-Si₅H₉PR₂, wherein R is lower [C₁-C₄] alkyl,phenyl or C₁-C₄-alkyl substituted phenyl) or dopant precursors in theformulation (e.g. tert-butyl phosphine), oxidized doped molecularsilicon ink formulations (e.g. oxidized versions of cyclic, linear orbranched silane oligomers or polymers (e.g., cyclo-Si₅O₅H₁₀) with dopantprecursors in the formulation (e.g. mono-, di- ortri-tert-butylphosphine or oxidized analogs thereof) or dopantsubstituents thereon, and glass forming formulations (e.g., so-calledsol-gel formulations) containing phosphorous and boron compounds (e.g.,organophosphates such as di-n-butylphosphate, etc.) and -borates (e.g.tri-t-butylborate, etc.). Suitable dielectrics also include compoundsand/or polymers containing phosphorous and oxygen (which may furtherinclude silicon, carbon, hydrogen and/or nitrogen), boron (which mayfurther include silicon, carbon, hydrogen, oxygen and/or nitrogen),arsenic and/or antimony (either of which may further include silicon,carbon, hydrogen and/or oxygen), etc. Exemplary phosphorous-containingdielectric precursors include:

-   -   inorganic oxophosphorous compounds and acids (e.g., P₂O₃, P₂O₅,        POCl₃, etc.);    -   phosphosilicates;    -   monomeric, dimeric and/or oligomeric phosphates (e.g. meta-        and/or polyphosphates);    -   phosphonates, phosphinates, and phosphines;    -   organic oxophosphorous compounds and acids (e.g., alkyl(aryl)        phosphates, phosphonates, phosphinates and condensation products        thereof); and    -   alkyl- and/or arylphosphonic and/or -phosphinic acids.

Exemplary boron-containing dielectric precursors include:

-   -   inorganic boron compounds and acids (e.g., boric acid, B₂O₃);    -   borosilicates, borazoles and polymers thereof;    -   boron halogenides (e.g., BBr₃);    -   boranes (e.g., B₁₀H₁₀), and sila- and/or azaboranes; and    -   organic boron compounds and acids (e.g. alkyl/aryl boronic acid,        borates, boroxines and borazoles, borane addition complexes        etc.).

Exemplary arsenic and/or antimony-containing dielectric precursorsinclude:

-   -   oxo- and/or aza-analogs of the above compounds, such as As₂O₃        and Sb₂O₃; and    -   arsinosilanes, such as cyclo-As₅(SiH₃)₅.

Thus, the source and drain terminals 6 in FIG. 1D may comprise (i) aGroup IVA element, a III-V compound semiconductor such as GaAs, or aII-VI (or chalcogenide compound) semiconductor such as ZnO or ZnS, and(ii) a dopant element. Preferably, the semiconductor comprises a GroupIV element (e.g., Si and/or Ge) and a dopant selected from the groupconsisting of B, P, As and Sb.

In various embodiments, the gate has a length of at least 0.1 microns,0.5 microns, 1 micron, or 2 microns. In one implementation, the minimumgate length is about 5 microns. In the present disclosure, andconsistent with gate dimensions as are known in the art, the gate lengthcorresponds to the gap width or spacing. The gate may have a width offrom about 1 μm to about 1000 μm or any range of values therein (e.g.,from about 2 μm to about 200 μm, or from about 5 μm to about 100 μm,etc.), and a thickness of from about 10 nm to about 10,000 nm or anyrange of values therein (e.g., from about 50 or 100 to about 5000 nm, orfrom about 200 to about 2000 nm, etc.). The source and drain terminals 6may have a thickness of from 10 to 1000 nm or any range of valuestherein (e.g., from 100, 200, or 250 Å to 10,000, 1000, or 500 Å),generally not to exceed the thickness of semiconductor layer 2.

Referring now to FIG. 1E, the gate metal 8 may be printed by depositinga suitable gate metal precursor (e.g., an ink containing metalnanoparticles and/or an organometallic compound(s), a doped molecularand/or nanoparticle-based silicon ink(s), a silicide precursor ink(s),etc.) at least into the gap defined by the printed doped glass pattern,then converting it to a gate metal. Use of doped silicon inks mayfurther require a high temperature anneal or laser irradiation to formpolycrystalline silicon and/or to activate the dopant to achievesufficient conductivity. Alternatively, a precursor for a seed layer(e.g., Pd) may be printed into the gap defined by the printed dopedglass pattern, and the gate metal (e.g., Ag, Au, Cu, Pd, Pt, etc.) maybe electroplated or electrolessly plated onto the seed layer. The seedlayer may require an activation step before the plating process.

Printing of the gate metal precursor may include ink-jetting, gravureprinting, offset lithography, etc. Furthermore (or alternatively),patterning the gate metal may include coating or printing the gate metalprecursor and locally exposing it to laser radiation such that itchanges its solubility characteristics in the exposed areas (see, e.g.,U.S. patent application Ser. No. 10/722,255, 10/749,876, and 11/203,563,filed Nov. 24, 2003, Dec. 31, 2003 and Aug. 11, 2005, the relevantportions of which are incorporated herein by reference). Upon washingaway the unexposed area, the irradiated gate metal precursor staysbehind to form the gate metal, optionally after an additional curing orannealing step (so-called “negative” patterning and developing).Alternatively, “positive” patterning and developing may be employed, inwhich the area exposed to radiation is washed away. These embodiments(including the positive patterning embodiment) may provide advantagesfor the patterning of high-resolution metal gates which may not bedirectly achievable with direct printing methods. In general, the gateconductor comprises a metal. However, with respect to the gate, “metal”includes doped polysilicon.

A metal-containing ink (and, for that matter, any other printable inkdisclosed herein) may be printed by essentially any conventionalprinting technology. For example, printing may comprise inkjet printing(“inkjetting”), screen printing, gravure printing, offset printing,flexography (flexographic printing), spray-coating, slit coating,extrusion coating, meniscus coating, microspotting, pen-coating,stenciling, stamping, syringe dispensing and/or pump dispensing themetal-containing ink in a predefined pattern. The ink may comprise orconsist essentially of the metal precursor material and a solvent. Themetal precursors that are generally compatible with printing or(selectively) plating may comprise organometallic compounds ornanoparticles (e.g., nanocrystals) of a metal such as titanium, copper,silver, chromium, molybdenum, tungsten, cobalt, nickel, gold, palladium,platinum, zinc, iron, etc., or metal alloys thereof, preferably silveror gold (or a metal alloy thereof). Such nanoparticles or nanocrystalsmay be conventionally passivated (e.g., with one or more surfactants),provided with one or more surface ligands (e.g., alkyl, aralkyl,alcohol, alkoxy, mercaptan, alkylthio, carboxylic acid and/orcarboxylate groups bound thereto, or H atoms adsorbed thereon), orremain unpassivated. Plating may comprise, in one example, laser writingor printing a seed layer of metal (e.g., Pd; see U.S. ProvisionalApplication No. 60/932,392, filed on May 30, 2007, the relevant portionsof which are incorporated herein by reference) using nanoparticlesand/or an organometallic compound of the metal, then selectivelydepositing (e.g., by electroless or electroplating) a bulk conductor(e.g., Co, Ni, Cu, etc.) onto the laser-written or printed seed layer.Alternatively, the ink may comprise or consist essentially of aconventional paste comprising a powder of one or more such metals oralloys thereof in a conventional binder.

The metal-containing ink may be dried by conventional and/or otherwiseknown processes. For example, metal precursor inks may be dried byheating the substrate containing the printed metal precursor ink thereonat a temperature and for a length of time effective to remove thesolvent and/or binder. Suitable temperatures for removing solvents froma printed ink may range from about 50° C. to about 150° C., or any rangeof temperatures therein (e.g., from about 80 or 100° C. to about 120°C.). Suitable lengths of time for removing solvents from a printed inkat such temperatures may ranges from about 1 seconds to about 10minutes, or any range of times therein (e.g., from about 10 or 30seconds to about 5 minutes, or about 1 minute to 3 minutes, etc.). Suchheating may take place on a conventional hotplate or in a conventionalfurnace or oven, optionally in an inert or reducing atmosphere.

The dried metal precursor from the ink may be further annealed at atemperature and for a length of time sufficient to improve itselectrical and/or physical properties (e.g., conductivity, morphology,electromigration and/or etch resistance, stress and/or surface strain,etc.) and/or its adhesion to the underlying gate oxide. When themetal-containing ink is globally (blanket) deposited or printed,annealing is generally conducted to form a metal film on which a resistmay be deposited for subsequent (laser) patterning (e.g., usingconventional photolithography or laser resist processing, and/or otherconventional processing. including developing, etching and/or strippingthe resist). Also, when directly laser writing a metal precursor inkresults in a patterned metal and/or metal precursor, annealing isgenerally performed to form a metal layer with improved conductivity,adhesion etc. Such annealing may comprise either annealing already-fusedmetal nanoparticles, or converting a patterned metal precursor layerinto a patterned metal (which may include a metal silicide or dopedpolysilicon). Suitable annealing temperatures generally range from about100° C. to about 500° C., or any range of temperatures therein (e.g.,from about 150° C. to about 400° C.), although a temperature of up to700° C. may be advantageous for formation of a metal silicide. Suitablelengths of time for annealing may range from about 1 minute to about 2hours, preferably from about 10 minutes to about 1 hour, or any range oftimes therein (e.g., from about 10 to about 30 minutes). Annealing maybe conducted in a conventional furnace or oven, optionally in an inertor reducing atmosphere. Thus, the present method may further comprisethe step of annealing the laser patterned metal gate sufficiently toimprove its electrical, physical and/or adhesive properties.

In one embodiment, the gate metal precursor ink may de-wet from theprinted doped glass pattern, which effectively confines it to the gapdefined by the doped glass pattern. The doped glass pattern may betreated (e.g., by plasma exposure, coating with a fluorinated layer orother material having similar de-wetting properties, etc.) beforedeposition of the gate metal precursor ink to ensure de-wetting.Similarly, the gate metal precursor ink and/or the doped glass precursormay contain additives which ensure de-wetting of the gate metalprecursor ink from the doped glass pattern. In an alternativeembodiment, the gate metal precursor wets the printed doped glasspattern so that it extends beyond the area of the gap between portionsof the doped glass pattern, and at least partially covers the dopedglass pattern. This embodiment may be advantageous for decreasing thegate-induced drain leakage in the device.

An exemplary process flow for making thin film transistors in accordancewith the present invention may therefore include the following steps:

-   -   Deposit or print lightly doped or undoped silane to form an        amorphous Si thin film    -   (Optional) Dehydrogenate amorphous Si    -   Crystallize lightly doped or undoped amorphous Si (e.g., by        [Excimer] laser treatment or furnace treatment)    -   Deposit, grow or otherwise form gate oxide (e.g., by thermal        oxidation)    -   Print or otherwise pattern source and drain areas by depositing        doped glass    -   Activate and/or diffuse dopants into source and drain areas        (e.g., by heat treatment)    -   (Optional) Deposit metal seed layer    -   Deposit gate metal    -   (Optional) Conventional gate metal annealing    -   Deposit passivation (e.g., oxide or nitride)

Oxide Formation after Printing Doped Glass

FIGS. 2A-E show an alternative process flow that advantageously combinesgate dielectric formation by thermal oxidation of polysilicon with drivein of the dopants from the doped glass either in situ or in one processstep. One important aspect of this second exemplary process, however, isto achieve gate oxide growth before significant dopant diffusion setsin.

A silicon film 12 is generally formed on substrate 11 by the sameprocess as in FIG. 1A, by printing or coating a molecular and/ornanoparticle based silicon ink, then converting it to a silicon film, orby conventionally depositing a silicon film (e.g. PECVD, LPCVD,sputtering, etc.). In either case, the silicon film is generallycrystallized by (UV) laser exposure, thermal furnace or RTA anneal(optionally in the presence of crystallization promoters such as Au, Ni,Al, etc.). Thereafter, referring to FIG. 2B, a doped glass 14 is printed(e.g. ink-jetting, gravure or offset litho printing, etc.) onto the(poly)silicon film. The layout of the printed doped glass pattern isessentially the same as the intended layout for the source-drainregions. Alternatively, the printed doped glass pattern may correspondto lightly doped extension regions (e.g., a lightly doped drain) in theTFT, in which case a second doped glass pattern may be subsequentlyprinted for forming the relatively heavily doped source/drain regions.The gap between the printed doped glass regions 14 generally defines theposition of the gate metal and the gate dielectric. The width of the gapmay range from 0.1-100 μm (preferably 1-10 or 1-5 μm), as describedabove. After printing, the doped glass pattern is optionally cured attemperatures low enough to ensure that substantially no dopant diffusesfrom the doped glass into the silicon film or into the gap defining thegate metal and dielectric. In one embodiment, a plurality of openings ismade in the doped glass pattern, exposing surfaces of thedopant-containing (e.g., lightly doped) regions of the semiconductorthin film layer.

Referring to FIG. 2C, gate dielectric 13 may be formed by thermalsilicon oxidation of the exposed poly-Si layer, printing or coating of asuitable dielectric precursor and converting it to a dielectric film, orliquid phase deposition of SiO₂ or other metal oxide (e.g., TiO₂, ZrO₂,HfO₂, etc.) or conventional silicon oxide and/or nitride depositionmethods (e.g., PECVD, LPCVD, sputtering of an elemental target in thepresence of an oxygen and/or nitrogen source, etc.). Preferably, siliconoxidation is employed.

Silicon oxidation in the exposed gate areas of the poly-Si film 12 canbe accomplished by heating the film in a suitable atmosphere (air, O₂,ozone, N₂O, wet or dry steam, or a combination thereof) to a temperaturehigher than 600° C. The maximum temperature is preferably lower than1000° C., more preferably lower than 900° C., to reduce, inhibit orprevent dopant diffusion from the doped glass into the channel region17.

Alternatively, liquid phase deposition of SiO₂ (or other metal oxide[s])may include growing an oxide layer in the exposed gate areas byhydrolysis of a saturated aqueous solution of hexafluorosilicic acid(H₂SiF₆) in the presence of boric acid at relatively moderatetemperatures. This approach avoids the relatively high temperatures forthermal growth of an oxide layer (e.g., from silicon) and therebyreduces or eliminates the risk of dopant diffusing from the doped glasspattern into the channel region 17, potentially shorting the transistor.

The gate dielectric film 13 in the present electronic device maycomprise any of the materials for the gate dielectric film describedherein. The gate dielectric film 13 may have a width and length slightlyless than the corresponding dimensions of the subsequently formed gatemetal layer when it is wet etched, but the two layers will havesubstantially the same width and length when dry etched. The gatedielectric film 13 may have a thickness of from 20 Å to 1000 Å or anyrange of values therein (e.g., from 30 to 300 or 400 Å, or from 50 to200 Å, etc.). Alternatively, a thicker gate dielectric layer (e.g., inthe range of from 500 to 2000 Å, and in one implementation, on the orderof about 150 Å) can be used, preferably using a material having a higherdielectric constant than silicon dioxide or aluminum oxide. In oneembodiment, the gate dielectric film 13 has a thickness greater than thethickness of the heavily doped source and drain terminals, largely tominimize the possibility that source and drain terminals may form anelectrical connection to gate metal layer. However, for high speedtransistors, generally a thin gate dielectric film is preferred.Typically, after the gate oxide 13 is formed, referring now to FIG. 2D,the temperature is raised sufficiently (e.g., to >800° C.) to diffuse(or “drive”) the dopant into the semiconductor film 12 to formsource/drain regions 16. Further silicon oxidation at the elevatedtemperature may occur concurrently with driving in the dopant. Theannealing temperature of the doped glass required to induce significantdopant diffusion is preferably higher than the temperature for forming adielectrically effective gate dielectric, but not higher than themaximum processing temperature of the substrate (e.g., for a stainlesssteel foil, a temperature of not more than 1100° C.). However, for ametal foil of a relatively low melting point material such as aluminum,the maximum processing temperature is not more than about 600° C., andthe gate oxide and dopant drive-in are performed at a relativelymoderate temperature (and in the case of dopant drive-in, perhaps withuse of a laser).

Thereafter, referring to FIG. 2E, gate metal 18 is formed insubstantially the same manner as gate metal 8 of FIG. 1E.

Passivation of the Doped Glass Layer

FIGS. 3A-C show a further alternative process flow that reduces theproblem of potential out-diffusion from the doped glasses during gatedielectric formation by depositing a barrier and/or passivation layer 25or generating a similar deactivated layer 25 on the doped glass. Thus,in a further embodiment, a thin dopant-depleted layer 25 is formed ontop of the printed doped glass pattern 24. This layer is intended toprevent dopant diffusion into the channel region 27 during dopantactivation. The formation of this layer can be accomplished in variousways (e.g., depositing an essentially undoped glass [acting as a buffer]or a silicon nitride film). Films which effectively getter typicaldopants (e.g. phosphorus or boron) may also be used. Alternatively, asurface layer depleted of dopant may be generated by exposing thepatterned doped glass film to hot water or water vapor, which extractssome of the dopant from the exposed surface(s) but leaves sufficientdopant in the bulk of the glass (particularly adjacent to the interfacewith underlying silicon 22) to be useful for source/drain doping.Furthermore, the thin passivation and/or dopant-depleted layer 25 may beformed by exposing the patterned glass film to conditions which changeits surface characteristics, such as to prevent significant dopantdiffusion into the channel, e.g. by exposing it to ozone, N₂O, water,etc.

Referring now to FIG. 3A, a passivation or barrier layer 25 (which canbe a dopant-depleted layer or an undoped passivation layer) can beapplied or formed prior to dopant drive-in at a temperature far belowthe temperatures typically employed to induce dopant diffusion. Such abarrier layer effectively prevents diffusion of the dopant from thedoped glass 24 at elevated temperatures into adjoining structures (e.g.,the top of the poly-Si channel 27 [see FIG. 3B] or the subsequentlyformed gate dielectric 23 and/or interlayer dielectric). Preferably, thepassivation/barrier layer 25 consists essentially of an undoped siliconoxide that does not present a barrier for the subsequent gate dielectricformation. A process capable of producing a barrier layer comprisesliquid phase deposition of SiO₂ from suitable precursor(s) (e.g., froman aqueous mixture of hydrofluorosilicic acid and boric acid), which canproduce thin layers of high quality (gate) dielectric at moderatetemperature. Alternatively, one may form passivation layer 25 byhydrolysis of a tetraalkylsiloxane or tetraalkoxysilane or conventionalleaching of the dopant from a surface layer of doped glass layer 24, inaccordance with known methods (such as by conventional extraction withwater).

Referring now to FIG. 3B, in the case of a gate dielectric formed byprinting or coating a suitable gate dielectric precursor and subsequentcuring/annealing (e.g., using liquid phase deposition or conventionalmethods), the lateral extent of the layer need not be confined to thegap between the doped glass pattern/structures 24. In fact, in someinstances, it may be preferable that at least parts of the gatedielectric layer 23 cover the surface of the doped glass patterncompletely or at least partially. In such cases, the gate dielectriclayer itself might also provide a barrier/passivation layer 25 andreduce, inhibit or prevent out-diffusion of dopants from the doped glasslayer 24 and/or source/drain patterns 26. Formation of source/drainregions 26 is substantially the same as for source/drain regions 16 inFIG. 1D.

In some cases, out-diffusion of dopant into the channel region 27 priorto oxidation may be desirable due to the enhanced oxidation rate of thedoped material. This enhanced oxidation rate would give a thicker gateoxide 23 at the edge of the channel 27. This thicker dielectric reducesthe electric field at the edge of drain 26, thereby reducing gateinduced drain leakage (GIDL).

Referring now to FIG. 3C, the gate metal 28 is printed depositing asuitable gate metal precursor (e.g., metal nanoparticles and/ororganometallic compound(s), doped molecular and/or nanoparticle-basedsilicon ink(s), silicide precursor ink(s), etc.) as described above intothe gap 25 defined by the printed doped glass pattern 24 and convertingit to the gate metal, as for gate metal 8 in FIG. 1E.

Formation of Source/Drain Contacts and Interconnections

The following process flows for the formation of source/drain contactsand interconnects can be used with any of the device structures and/orprocesses described above.

Gate as Mask for Source/Drain Contact Etch

FIGS. 4A-D show an exemplary embodiment of a process for formingsource/drain contacts and an interlayer dielectric (ILD) on the basicTFT structures of FIGS. 1-3. Referring to FIG. 4A, this exemplaryprocess flow uses a printed metal gate 140, partially covering the dopedglass pattern 130, as a mask for etching the doped glass pattern 130 toexpose the contact areas of the source/drain regions 112 and 114. Thisembodiment ensures a relatively close distance of the source/draincontacts (thereby reducing the resistance) and enables an optionalsilicide on the source/drain contacts, as no organic ILD (interlayerdielectric) is present over the source/drain regions 112/114.Furthermore, extending the gate metal 140 beyond the area of the gatedielectric 120 and leaving some of the doped glass 130 under the gatemetal 140 can reduce gate-induced drain leakage.

Source/drain regions 112/114 and channel 116 are formed on substrate 100similarly to source/drain regions 6 and channel 7 of FIG. 1D. Undoped orlightly doped semiconductor layer portions 110 remain after drive-in ofdopant from doped glass layer 130. Doped glass layer 130 remains afterformation of source/drain regions 112/114. Gate dielectric 120 is formedsimilarly to gate dielectric 3 of FIG. 1C or gate dielectric 13 of FIG.2C. For example, oxide layer 122 may be formed during oxidation ofexposed surfaces of the semiconductor layer (e.g., 110) not covered bydoped glass layer 130.

As shown in FIG. 4B, etching of the doped glass pattern 130 and exposeddielectric layer 122 is accomplished by exposure to one or more suitableetchants, including (but not limited to) HF-based wet etchants (e.g.buffered oxide etch [BOE]), native oxide etch [NOE], conventional padetch, a conventional pyridine:HF etchant solution, etc.), HF-based or-producing vapors or gases, plasma etching, etc. The etchant is chosensuch that the etch rate of the (gate) dielectric 122 and doped glass 130is sufficiently larger than the etch rate of the underlying silicon(e.g., layers 110, 112 and 114) and metal gate layer 140 to enablesubstantially complete removal of the doped glass without anysubstantial removal of the silicon or gate metal.

After etching and an optional cleaning step (not indicated), referringnow to FIG. 4C, an interconnect metal 150/152 is printed on the exposedsource/drain contacts. Although not shown in FIG. 4C, the interconnectmetal is also printed onto the exposed metal, but in a “pad” region notin the plane of the page, as is known in the art. In one embodiment, theinterconnect metal features 150 or 152 may also contact the gate metal140 to form a diode-configured transistor (not shown). The printedinterconnect metal is used to connect transistors within the same layerand/or to provide the lower contact area for a via structure. Theresistance of the interconnect metal is preferably lower than 10Ohm/square.

To ensure good contact, the structure of FIG. 4C may furthermore beannealed to form a silicide at the interface of the interconnect metal150/152 with silicon, or throughout the entire film thickness of thecontact areas between the interconnect metal 150/152 and the underlyingsilicon 112/114. Suitable silicide forming metals include but are notlimited to Al, Ni, Pd, Pt, Mo, W, Ti, Co, etc. The interconnect metalmay be chosen from such silicide forming metals. Alternatively, theinterconnect metal precursor ink may contain additives which formsilicides (e.g., silver inks doped with Ni organometallics) have beenobserved to lower the contact resistance between the interconnect150/152 and doped silicon source/drain regions 112/114. However, theadditive (e.g., Ni) may segregate to the silicon interface and/or form asilicide.

After printing the interconnect metal 150/152 (which may also be formedby more conventional processes, such as sputtering and photolithography,but printing is preferred), and referring now to FIG. 4D, an interlayerdielectric 160 is printed to cover any exposed active areas (e.g., thegate 140 and source/drain regions 112/114), but leaving via holes162/164 in appropriate areas (e.g., over interconnect 150/152). Theinterlayer dielectric precursor may comprise a glass-forming formulation(e.g., a spin-on-glass formulation, such as a conventionalorganosilicate or organosiloxane), an organic dielectric (e.g., apolyimide, poly(benzocyclobutene) [BCB], etc.), an oxidized siliconprecursor (e.g., an oxidized silane, such as Si₅H₅(OH)₅, etc.), or amolecular and/or nanoparticle based silicon formulation (e.g., a silaneink) which is oxidized after printing.

Printing the interlayer dielectric 160 may comprise ink-jetting,gravure, offset printing, etc., similar to the other printable inksdisclosed herein. Alternatively, patterning the interlayer dielectricmay comprise printing or depositing an interlayer dielectric (e.g. UV-and/or IR-sensitized polyimide), and exposing it to radiation (e.g. IR,visible, or UV radiation) to change its solubility characteristic(s) inthe irradiated areas. Exposing this layer to a suitable etchant orsolvent (e.g., a developer) may remove the exposed (positive) orunexposed (negative) area of the interlayer dielectric forming viaholes.

In an alternative embodiment, a sacrificial material may first beprinted in locations corresponding to the locations of the later formedvia holes in the interlayer dielectric. Then, an interlayer dielectricprecursor as outlined above is printed or globally (blanket) deposited.Upon curing the interlayer dielectric precursor, the sacrificialmaterial in the via areas may decompose, forming the via holes. Othermeans of removing the sacrificial material from the via hole areenvisioned (e.g., selective etching, etc.). The circuit is completed byprinting an interconnect metal connecting the respective contact pads inthe open via-holes (see, e.g., FIGS. 7B-C and 8B-C). The same techniquesand materials as outlined above can be utilized.

Conductors in communication with one of the source/drain terminals orthe gate terminal may also be coupled to or continuous with another oneof the conductors. For example, in a diode-configured transistor, aconductor may be in electrical communication with one source/drainterminal and the gate. In a capacitor-configured transistor, a conductormay be in electrical communication with both source/drain terminals.Alternatively, a thin dielectric layer may be formed over a source/drainterminal, and a conductor capacitively coupled to the underlyingsource/drain terminal may be formed thereover.

ILD as a Mask for Source/Drain Contact Etch

FIGS. 5A-E show structures formed during an alternative process formaking the present TFT. Referring to FIG. 5A, channel 210, firstsource/drain region 212, second source/drain region 214, and undopedsemiconductor (e.g., Si) regions 216 are formed on substrate 200 asdescribed herein. Doped glass 230 and conductive gate metal 240 areprinted on or over substrate 200 as described herein, and gatedielectric 220 and thermal oxide (or other dielectric) 222/224 areformed as described herein.

Similar to the process of FIGS. 4A-E, the process of FIGS. 5A-E utilizesa gate 240 which is essentially confined to the area between printeddoped glass regions 230 (as is gate dielectric 210) and does not coverthe doped glass pattern 230. However, in this embodiment, a firstinterlayer dielectric 245 is deposited to protect the gate metal 240 andgate dielectric 220 during the etch of the doped glass pattern 230 toexpose the source/drain regions 212/214 or contacts thereon (not shown).

In FIG. 5B, first interlayer dielectric 245 is printed such that itcompletely covers the gate metal and dielectric 220, and at leastpartially but not completely the doped glass pattern 230. The precursorink for first interlayer dielectric 245 may comprise glass-formingformulations (e.g., spin-on-glass formulations [e.g. silicates ororganosiloxanes]), organic dielectrics [e.g., polyimide, BCB, etc.],oxidized silicon precursors [e.g., an oxidized silane such asSi₅O₅H₁₀]), or molecular and/or nanoparticle based silicon formulationswhich are oxidized after printing, as described elsewhere herein.

Printing the first interlayer dielectric 245 may comprise inkjetting,gravure, offset printing, etc. Alternatively, patterning the interlayerdielectric may comprise printing or depositing an interlayer dielectric(e.g. UV- and/or IR-sensitized polyimide), and exposing it to radiation(e.g., IR, visible, or UV radiation) to change its solubilitycharacteristic(s) in the irradiated areas. Exposing this layer to asuitable etchant or solvent (e.g., developer) will remove the exposed(positive) or unexposed (negative) area of the interlayer dielectricforming via holes.

Subsequently, as shown in FIG. 5C, the doped glass pattern 230 andthermal oxide regions 222 and 224 are etched sufficiently to removethermal oxide regions 222 and 224 and expose the doped source/drainregions 212/214. Etching the exposed doped glass pattern 230 and theexposed thermal oxide 222/224 is accomplished by exposure to a suitableetchant, including but not limited to HF-based wet etchants (e.g. BOE,NOE, pad etch, pyridine:HF, etc.), HF-based or -producing vapors orgases, plasma etching, etc., and generally for a length to timesufficient to remove thermal oxide regions 222 and 224, but keep someportion of the first interlayer dielectric 245 remaining over the gatemetal 240. In many embodiments, some portion of doped glass pattern 230also remains over the doped source/drain regions 212/214. The etchantmay be non-selective between the doped glass pattern 230 and the firstinterlayer dielectric 245, the doped glass pattern 230 and thermal oxideregions 222/224, or all three materials (i.e., the doped glass pattern230, the first interlayer dielectric 245, and thermal oxide 222/224),but the etchant generally is chosen such that the etch rate of the dopedglass 230 and thermal oxide 222/224 is sufficiently larger than the etchrate of the underlying semiconductor (e.g., source/drain regions 212/214and undoped semiconductor regions 216) to enable substantially completeremoval of the thermal oxide 222/224 without any substantial removal ofthe underlying semiconductor.

Thus, depending on the etch time, the etchant may remove only relativelynarrow portions of the doped glass pattern 230, exposing only a smallboundary or area of the source/drain regions 214/216. In this case, allmaterials mentioned above are suitable for the first interlayerdielectric 245, as long as its thickness is sufficient to protect thegate metal and dielectric given its potentially low etch selectivityrelative to the doped glass 230.

Alternatively, and referring now to FIG. 6A, the etch time may be chosensuch that essentially most of the doped glass pattern is removed and asmall amount of doped glass (or other insulator) 232 remains adjacent tothe gate metal 240 and gate dielectric 220. In this case, the interlayerdielectric 245 is chosen such that it has a negligible etch ratecompared to the etch rate of the doped glass pattern. For example, anorganic insulator (e.g. polyimide, BCB etc.) may be chosen in thisembodiment. In those embodiments employing a passivation and/ordopant-depleted layer on the printed doped glass pattern (see, e.g.,FIGS. 3A-3C), the undoped passivation/dopant-depleted layer can beselected such that it has a negligible etch rate compared to the etchrate of the doped glass pattern in the presence of the doped glassetchant.

Subsequently, the doped glass pattern 230 (see FIG. 5B) is etched toexpose the source/drain regions 212/214 and leave doped glass“remainders” 232 (see FIG. 6A). Etching the doped glass pattern and theexposed thermal oxide regions 222/224 is accomplished substantially asdescribed herein, but the etchant is chosen such that the etch rate ofthe doped glass and thermal oxide 222/224 is selective with respect tothe interlayer dielectric layer 245 (e.g., the etch rate ratio of thedoped glass and thermal oxide 222/224 to the interlayer dielectric layer245 is sufficiently large to enable substantially complete removal ofthe exposed doped glass without any substantial removal of theinterlayer dielectric layer 245). In embodiments where the doped glasspattern 230 comprises or is based on a doped silicon oxide, andsource/drain regions 212/214 and undoped semiconductor regions 216comprise or consist essentially of silicon, the interlayer dielectriclayer 245 may comprise silicon nitride.

Referring now to FIGS. 5D and 6B, after etching the doped glass 230 andthermal oxide 222/224, the substrate may be (optionally) cleaned, and aninterconnect metal 250/252 is printed on the exposed source/drainregions 212/214, respectively. As described elsewhere herein, theinterconnect metal 250/252 may also contact the gate metal 240 (notshown). The printed interconnect metal 250/252 is used to connecttransistors within the same layer and/or to provide a lower-resistancecontact area for an overlying via structure. If the interlayerdielectric 245 is chosen to be compatible with subsequenthigh-temperature processing (e.g., a silicate, silicon nitride, etc.), ametal silicide can be formed at the interface between the interconnectmetal 250/252 and source/drain region 212 and undoped semiconductor 216on one side of the gate metal 240 and source/drain region 214 andundoped semiconductor 216 on the other side. The resistance of theinterconnect metal is preferably lower than 10 Ohm/square.

Printing and forming the interconnect metal may include printing asuitable interconnect metal precursor (e.g. metal nanoparticles ororganometallic compound(s), silicide precursor ink(s), etc.) andconverting it to the interconnect metal, largely as described herein.Alternatively, a precursor for a seed layer can be printed on thecontact areas and converted to a seed layer, then the interconnect metal(e.g., Ag, Au, Cu, Pd, Pt, etc.) can be electroplated or electrolesslyplated on the seed layer. The seed layer may require an activation stepbefore the plating process.

Alternatively, patterning of the interconnect metal may include coatingor printing the interconnect metal precursor and locally exposing it tolaser radiation such that the solubility characteristics of the printedinterconnect metal precursor changes in the exposed areas. Upon washingaway either the exposed or unexposed area (preferably the unexposedarea), the irradiated interconnect metal precursor stays behind to formthe interconnect metal, optionally after an additional curing orannealing step. This embodiment may provide advantages for thepatterning of high-resolution metal interconnect which may not directlybe achievable with direct printing methods.

To ensure good contact, the structure may furthermore be annealed toform a silicide at the interface or throughout the entire film thicknessof the contact areas between the interconnect metal and the silicon.Thus, in such an embodiment, the interlayer dielectric protecting thegate metal 240 during the etch of the doped glass pattern 230 iscompatible with the silicidation temperature.

Referring now to FIGS. 5E and 6C, after printing the interconnect metal,a second interlayer dielectric 260/262/264 is printed over the gate 245and source/drain regions 212/214, but leaving via holes 280 inappropriate areas for contact with an overlying level of metallization.The precursor for interlayer dielectric 260-264 may comprise the same orsimilar formulations as for first interlayer dielectric 245 (e.g.,spin-on-glass formulations such as silicates or siloxanes, organicdielectrics such as polyimides, BCB, etc., oxidized silicon precursorssuch as oxidized silanes, etc.), or molecular and/or nanoparticle basedsilicon or aluminum formulations which are oxidized and/or nitridedafter printing.

A Printed ILD as a Mask for Doped Glass Etching

The process flows of FIGS. 7A-7D and 8A-8D use a printed interlayerdielectric pattern 350/352/354 as a mask for etching the doped glass 330in order to expose the source/drain regions 312 and 314. The processflows of FIGS. 7A-7D and 8A-8D differ from each other primarily in theetch selectivity of the doped glass pattern 330 relative to the firstinterlayer dielectric 350/352/354 (or, as shown in FIGS. 8A-8D, firstinterlayer dielectric 350′/352′/354′). In the process of FIGS. 7A-7D,the etch is selective, thereby enabling a thinner printed interlayerdielectric pattern 350/352/354 relative to the process of FIGS. 8A-8D.In the process of FIGS. 8A-8D, the etch is non-selective, therebyenabling a wider variety of possible materials for printed interlayerdielectric pattern 350′/352′/354′ relative to the process of FIGS.7A-7D.

Referring to FIGS. 7A and 8A, a first interlayer dielectric 350/352/354(or 350′/352′/354′) can be printed such that it completely covers thegate metal 340 and the regions of substrate 300 that would otherwise beexposed, and partially (but not completely) covers the doped glasspattern 330 as well as the semiconductor island edges (e.g., 316). Theprecursor for the first interlayer dielectric 350/350′ through 354/354′may comprise any of the dielectric-forming formulations describedherein, as well as a nitride or oxynitride of silicon and/or aluminum.The interlayer dielectric may be printed or patterned as describedherein.

Subsequently, and as shown in FIGS. 7B and 8B, the exposed doped glasspattern 330 and thermal oxide 322/324 are etched to expose thesource/drain regions 312/314 in areas substantially not covered by thefirst interlayer dielectric 350/352/354 (or 350′/352′/354′). The dopedglass pattern 330 is etched as described elsewhere herein. In theprocess of FIG. 7B, the etchant is generally chosen such that the etchrate of the doped glass 330 and thermal oxide (or other dielectric)322/324 is sufficiently larger than the etch rate of the firstinterlayer dielectric 350/352/354 and the underlying source/drainregions 312/314 to enable substantially complete removal of the dopedglass 330 without any substantial removal of the first interlayerdielectric 350/352/354 or source/drain regions 312/314. In the processof FIG. 8B, the etchant is generally chosen such that the etch rate ofthe doped glass 330 and thermal oxide 322/324 is close to orsubstantially the same as the etch rate of the first interlayerdielectric 350′/352′/354′, but relatively higher than the etch rate ofthe underlying source/drain regions 312/314. Depending on the choice ofthe interlayer dielectric and its thickness, the removal of the dopedglass pattern may lead to undercut structures 332 (FIG. 7B) or 332′(FIG. 8B). However, when the etch selectivity of the doped glass to thefirst interlayer dielectric is low (e.g., as shown in FIG. 8B), theremay be appreciable thinning of the first interlayer dielectric (comparethe etched first interlayer dielectric 356/357/358 in FIG. 8B to theprinted/patterned first interlayer dielectric 350′/352′/354′ in FIG.8A). In such a case, the printed/patterned first interlayer dielectric350′/352′/354′ may have a thickness that is greater than the thicknessof doped glass 330 (e.g., by ≧1.5×, ≧2×, ≧3×, ≧5×, or ≧10×). Asillustrated in FIG. 8B, this arrangement can avoid the formation ofundercut structures 332.

In either case, the etch time is chosen such that most (but not theentire) doped glass layer is removed from over the source/drain regions312/314. Specifically, a small amount of doped glass 332 adjacent to thegate metal 340 and gate dielectric film 320, covered by the firstinterlayer dielectric 350 or 350′, remains.

After etching and an optional cleaning step (not shown), an interconnectmetal 360/362 can be printed on the exposed source/drain regions312/314, as shown in FIGS. 7C and 8C. Furthermore, this interconnectmetal may also contact the gate metal (not shown). The printedinterconnect metal may be used to connect transistors within the samelayer and/or to provide a lower contact area for a via structure. If theinterlayer dielectric is compatible with subsequent high-temperatureprocessing (e.g., a silicate, silicon nitride, etc.), silicidation ofthe source/drain contact may be possible after depositing theinterconnect metal 360/362. The resistance of the interconnect metal ispreferably lower than 10 Ohms/square. Printing and forming theinterconnect metal may be performed as described elsewhere herein.

After printing the interconnect metal 360/362, as shown in FIGS. 7D and8D, a second interlayer dielectric 370 may be printed covering theexposed active areas (e.g., the gate and source/drain regions), butleaving via holes 380 in appropriate areas. The precursor for theinterlayer dielectric 370 may comprise the same dielectric-formingformulations and other materials as described herein for interlayerdielectrics.

In certain embodiments of the present process (not necessarily limitedto FIGS. 7A-8D), laser patterning may comprise the substeps ofdepositing a resist material on the blanket deposited metal-containinglayer, selectively irradiating portions of the resist material with abeam of light from a laser having (i) a predetermined width and/or (ii)a predetermined wavelength or wavelength band absorbed by the resist (orby an absorptive dye in the resist), developing the selectivelyirradiated resist with a developer to leave a pattern corresponding tothe structure being formed (in the case of FIGS. 7A-8D, gate metal 340and/or interconnect 360/362; note that these steps apply to bothpositive and negative resists), removing those portions of the blanketdeposited material not corresponding to the desired or predeterminedpattern (typically by dry or wet etching), and removing the remainingresist material. Preferably, the light has a wavelength in the infrared(IR) band, although it could also comprise a wavelength or wavelengthband in the ultraviolet (UV) and/or visible band of the spectrum. Insuch a case, the resist (or dye) absorbs and/or is sensitive to thatwavelength or band of light, and the light beam is focused on ordirected at the desired or predetermined portions of the resist.Exemplary embodiments are disclosed in U.S. patent application Ser. No.11/203,563, filed on Aug. 11, 2005, the relevant portions of which areincorporated herein by reference.

Alternatively, a semiconductor layer (e.g., having characteristics ofthe transistor channel 7, 17, 27, 116, 210, or 310, such a dopant levelor concentration) may be formed by printing or coating the substratewith a doped or undoped semiconductor ink while simultaneouslyirradiating the ink/substrate. In one implementation, the processcomprises spin-coating the ink containing a semiconductor precursor ontoa substrate, while irradiating the ink with ultraviolet light during asubstantial part of the spin-coating step. This technique (which, in thelatter implementation, is sometimes known as “UV spin-coating”) isdescribed in greater detail in co-pending U.S. patent application Ser.No. 10/789,274, filed on Feb. 27, 2004, the relevant portions of whichare incorporated herein by reference. In another implementation,printing (which may include simultaneous or immediately subsequent UVirradiation) comprises inkjetting or gravure, flexographic, screen oroffset printing a doped or undoped semiconductor ink in locations on thesubstrate corresponding to active transistor regions (or otherdeposition technique for depositing material in selected areas of thesubstrate). In either case, the semiconductor layer generally has anamorphous morphology following deposition (with optional substantiallysimultaneous irradiation) and curing, and before further processing, itis generally crystallized (e.g., by heating or by laser irradiation;see, e.g., U.S. patent application Ser. Nos. 10/950,373 and 10/949,013,each of which was filed on Sep. 24, 2004, the relevant portions of whichare incorporated herein by reference). In many cases, suchcrystallization will also activate at least some of the dopant.

CONCLUSION/SUMMARY

The present invention advantageously provides a low cost method formaking printed self-aligned top-gate TFTs having reliable, commerciallyacceptable electrical characteristics (e.g., on/off speeds and ratios,carrier mobilities, V_(t)'s, etc.). The invention advantageouslyprovides (1) results similar to structures formed by more conventionalapproaches, but at a much lower cost and at a much higher throughput (onthe order of hours to days, as opposed to weeks to months) thanconventional process technology, and (2) TFTs with higher resolutionpatterning capability and similar or higher throughput as compared toconventional graphic arts printing technology (e.g., inkjetting).

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the Claims appended hereto and theirequivalents.

The invention claimed is:
 1. A method of forming a thin film transistor(TFT), comprising: a) forming a semiconductor thin film layer; b) priorto forming a gate electrode of the TFT, printing a doped glassformulation in a pattern on or over the semiconductor thin film layer,the printed doped glass pattern having a gap therein exposing a portionof the semiconductor thin film layer and defining at least one dimensionof at least part of the gate electrode; c) after printing the dopedglass pattern, forming the gate electrode in the gap, the gate electrodecomprising a gate dielectric film and a gate conductor thereon; and d)diffusing a dopant from the doped glass pattern into the semiconductorthin film layer.
 2. The method of claim 1, wherein forming thesemiconductor thin film layer comprises printing a liquid-phase inkcomprising a semiconductor-containing precursor onto a substrate.
 3. Themethod of claim 1, wherein the doped glass formulation is printed ontoor over regions of the semiconductor thin film layer corresponding tosource and drain terminals of the TFT.
 4. The method of claim 1,comprising printing the doped glass formulation on the semiconductorthin film layer, then forming the gate dielectric film on at least anexposed surface of the semiconductor thin film layer in the gap.
 5. Themethod of claim 1, wherein diffusing the dopant comprises heating thedoped glass pattern and the semiconductor thin film layer to atemperature and for a length of time sufficient to form a source/drainstructure in the semiconductor thin film layer.
 6. The method of claim1, wherein the step of printing the doped glass formulation comprisesprinting an ink comprising a doped glass precursor on or over thesemiconductor thin film layer, then converting the doped glass precursorink to a doped glass.
 7. The method of claim 1, further comprisingremoving at least portions of the doped glass pattern, sufficiently toexpose surfaces of the semiconductor thin film layer containing diffuseddopant.
 8. The method of claim 7, wherein removing portions of the dopedglass pattern leaves remaining portions of the doped glass pattern. 9.The method of claim 7, further comprising forming a conductiveinterconnect structure on the exposed surfaces of the semiconductor thinfilm layer containing diffused dopant.
 10. The method of claim 7,further comprising forming an interlayer dielectric film over the dopedglass pattern and the gate electrode.
 11. The method of claim 1, furthercomprising forming a passivation layer or a dopant-depleted layer on anexposed surface of the doped glass pattern.
 12. A method of forming athin film transistor (TFT), comprising: a) forming a semiconductor thinfilm layer; b) prior to forming a gate electrode of the TFT, printingdoped glass on or over the semiconductor thin film layer in a patternthat forms a gap in the doped glass defining a channel region and across-sectional width of at least part of the gate electrode; c) afterprinting the doped glass in the pattern, forming the gate electrode onor over the channel region and in the gap, the gate electrode comprisinga gate dielectric film and a gate conductor thereon; d) diffusing adopant from the doped glass pattern into the semiconductor thin filmlayer; e) forming a patterned interlayer dielectric film over the dopedglass pattern and the gate electrode, the patterned interlayerdielectric film having an exposed upper surface; and f) etching thedoped glass pattern using the exposed patterned interlayer dielectricfilm as a mask.
 13. The method of claim 12, wherein etching the dopedglass pattern exposes surfaces of the semiconductor thin film layercontaining diffused dopant.
 14. The method of claim 13, wherein etchingthe doped glass pattern leaves remaining portions of the doped glasspattern.
 15. The method of claim 13, further comprising forming aconductive interconnect structure on the exposed surfaces of thesemiconductor thin film layer containing diffused dopant.
 16. The methodof claim 15, wherein forming the conductive interconnect structurecomprises printing an ink containing a metal precursor and convertingthe metal precursor to a metal.
 17. The method of claim 15, furthercomprising printing a second interlayer dielectric film over exposedactive areas.
 18. The method of claim 12, wherein forming thesemiconductor thin film layer comprises printing a liquid-phase inkcomprising a semiconductor-containing precursor onto a substrate andconverting the semiconductor-containing precursor to the semiconductorthin film.
 19. The method of claim 18, wherein thesemiconductor-containing precursor comprises a silane or siliconnanoparticles.
 20. The method of claim 12, wherein forming the patternedinterlayer dielectric film comprises printing an ink containing adielectric precursor and converting the dielectric precursor to thepatterned interlayer dielectric film.
 21. The method of claim 12,wherein the etching step comprises wet etching exposed surfaces of thedoped glass pattern with a HF-based wet etchant.
 22. The method of claim12, wherein the doped glass pattern is printed onto or over regions ofthe semiconductor thin film layer corresponding to source and drainterminals of the TFT.
 23. The method of claim 12, comprising printingthe doped glass pattern on the semiconductor thin film layer, thenforming the gate dielectric film on at least an exposed surface of thesemiconductor thin film layer in the gap.
 24. The method of claim 12,wherein diffusing the dopant comprises heating the doped glass patternand the semiconductor thin film layer to a temperature and for a lengthof time sufficient to form a source/drain structure in the semiconductorthin film layer.
 25. The method of claim 12, wherein the step ofprinting the doped glass pattern comprises printing an ink comprising adoped glass precursor on or over the semiconductor thin film layer, thenconverting the doped glass precursor to a doped glass.
 26. A method offorming a thin film transistor (TFT), comprising: a) forming asemiconductor thin film layer; b) forming a gate dielectric film on anentire surface of the semiconductor thin film layer; c) prior to forminga gate electrode of the TFT, printing a doped glass formulation in apattern on or over the gate dielectric film, in a manner leaving a gapin the doped glass pattern exposing a portion of the gate dielectricfilm and defining a channel region and at least one dimension of atleast part of the gate electrode; d) after printing the doped glasspattern, forming the gate electrode in the gap, the gate electrodefilling a width of the gap and comprising the gate dielectric film and agate conductor thereon; and e) diffusing a dopant from the doped glasspattern into the semiconductor thin film layer.
 27. The method of claim26, comprising forming the gate conductor on the gate dielectric film,at least in the gap.
 28. The method of claim 27, wherein forming thegate conductor comprises printing an ink comprising a gate conductorprecursor on the gate dielectric film.